Motor-cycle radial tyre

ABSTRACT

A motor-cycle tire ( 1 ) comprising a ground contacting tread ( 2 ) reinforced between its edges ( 3,4 ) by a breaker assembly and having in its normally inflated fitted condition a camber value C/L of between 0.5 and 0.7, a reinforcing carcass ply ( 14 ) extending radially inside the breaker assembly and between two bead regions ( 10,11 ) and wrapped in each bead region around an annular bead core ( 12,13 ) from the axial inside to the outside to form carcass ply turn-ups ( 15,16 ) and between the tread edges ( 3,4 ) and bead regions ( 10,11 ), tire sidewalls ( 8,9 ) wherein the breaker assembly comprises two breaker plies ( 5,7 ) comprising reinforcing cords oppositely inclined with respect to the circumferential direction of the tire characterized in that the tread comprises two different rubber compounds, the first rubber compound extending in a continuous first layer between the tread edges ( 3,4 ) and the second rubber compound being disposed in a second layer radially outwardly of the layer of the first rubber compound to form the tread running surface in the central portion of the tread.

[0001] This invention relates to radial motor-cycle tires and in particular but not exclusively to high performance or race motor-cycle tires.

[0002] Such tires utilize very wide treads which in transverse cross-section are sharply curved to provide good contact with the road surface when the motor-cycle is steeply banked in cornering. Maintenance of a consistent ground contact area or ‘tire footprint’ under all conditions is a major problem in determining general vehicle handling. Of particular importance in race motor-cycle tires of radial construction is the provision of high cornering power with the good stability to maximise cornering speeds under race conditions.

[0003] Present radial motor-cycle race tires have short sidewalls which extend to the tread edges radially and axially outwardly from the tire beads. The beads provide engagement to the wheelrim on tapered bead seats. The sidewalls are reinforced by radial carcass plies which when tensioned by the inflation pressure act together with sidewall geometry to provide location of the curved tread regions to withstand cornering forces.

[0004] The sharply curved tread region of the tire is specially reinforced by a reinforcing breaker to give the required structural rigidity to allow for banking over of the motorcycle when cornering whilst giving sufficient flexibility to allow localised tread flattening in the ground contact patch for good road grip.

[0005] Also in the art EP-A-774 367 proposes the use of two different tread compounds for a motorcycle race tire where one side of the tread is provided with high grip compound and the other side with a harder compound. The construction is prepared for race circuits where the majority of bends are in the same direction.

[0006] Also Japanese Patent Publication 61-211106 discloses a motorcycle tire with a tread formed with three separate compound zones each of which is at full tread thickness.

[0007] Such tires however, do not provide fully optimum performance with regard to tread durability. It therefore an object of the present invention is to improve the properties of such tires.

[0008] Accordingly the present invention provides a motorcycle tire having the features set out in claim 1.

[0009] By camber value is meant the ratio C/L between the radial distance C from the centre to the edge of the tire tread and the axial distance L between the centre and edge of the tread.

[0010] Further aspects of the present invention will become apparent from the description of the following embodiments in conjunction with the attached diagrammatic drawing in which:

[0011]FIG. 1 shows in cross-section a 170/60 R17 radial motor-cycle tire intended for high speed racing.

[0012] The tire 1 of FIG. 1 comprises a pair of sidewalls 8 and 9 terminating in bead regions 10 and 11. Each bead region is reinforced by an inextensible annular bead core 12 and 13. Extending between each bead region is a tire carcass reinforcement ply 14 which is anchored in each bead region by being turned around the respective bead core 12,13 laterally from the inside to the -outside to form a ply turn-up 15,16. The carcass reinforcement ply 14 comprises a single ply of tire fabric laid with nylon cords substantially radially. Each bead region 10,11 further comprises a hard rubber apex member 17,18 which is anchored to each respective bead core 12,13 and extends taperingly radially outwardly.

[0013] The carcass ply fabric of the present tire may also comprise polyester, rayon or aramid cords. Further whilst a single ply carcass of cords at substantially 900 may be particularly advantageous in the case of tires for the rear wheel of the motorcycle, for the front wheel a tire with two plies of cords crossed at an angle of 70°-88° may be advantageous.

[0014] The tire 1 has a camber value of 0.6 and comprises a convex tread region 2, having tread edges 3,4, reinforced by a breaker assembly. The width TW of the tread is 220 mm measured along the outer surface. The breaker assembly comprises two breaker plies 6 and 7 each of which comprises Kevlar (Registered Trade Mark) aramid cord tire fabric each of 2/165 TEX. The cords in each of the breaker plies 6 and 7 are oppositely inclined to each other at an angle of 25° to the circumferential direction of the tire. The radially inner breaker ply 7 has a width Bi of 200 mm and is narrower than the radially outer breaker ply 5 which has a width Bo of 220 mm. The breaker plies may also comprise steel cords.

[0015] As evident from FIG. 1 the tread of the tire comprises two distinct layers of rubber. The first rubber layer RL1 extends continuously between the tread edges 3 and 4.

[0016] In the central tread portion a second rubber layer RL2 is disposed radially outward of the first rubber layer RL1 and centred on the tire circumferential centreline C/L.

[0017] The tread ground contacting surface comprises in the centre the rubber compound of the second rubber layer and to either side in the lateral regions a different rubber compound of the first rubber layer RL1. The portion of the first rubber layer RL1 extending radially inward of the second rubber layer has a thickness of 2 mm.

[0018] At the boundary between the first and second rubber layer is disposed an interfacial fabric layer which comprises in this embodiment a rubberised cut fabric ply. This ply comprises nylon cords disposed in mutually parallel configuration and at an angle of between 18°-30° preferably 25° to the tire circumferential direction. In accordance with the invention the interfacial fabric may comprise other textile materials common in the art.

[0019] The second layer of rubber RL2 has a width S which is 40% of the tread width TW. According to the invention this width may be in the range of 30%-50%. If the second rubber layer width exceeds 50% of the tread width then the second rubber compound extends too far into the lateral regions of the tread and the cornering performance of the tire deteriorates.

[0020] The compound properties of the rubber of the first layer RL1 are such that the material provides good grip on cornering whilst those of the rubber of the second layer RL2 are to give low temperature generation in straight running.

[0021] Preferably the properties of the rubber layer of RL1 are characterised by a hardness value at 100° C. of between 40-49 Shore ‘A’ and a tensile modulus at 100° C. and 300% extension of less than 5.0 MPa.

[0022] In contrast the properties of the second rubber layer RL2 are preferably a hardness value at 100° C. of not less than 50 Shore ‘A’ and a tensile modulus at 100° C. and 300% extension of not less than 5.0 MPa.

[0023] Example formulations of rubber compounds suitable for the invention are shown in Table 1.

[0024] The formulation A is a compound suitable for the softer first rubber layer RL1 whilst compound B is suitable for the harder second rubber layer RL2.

[0025] The polymer type can be emulsion or solution polymerised styrene butadiene (SBR) and may contain 20-50% styrene, preferably 25-45%.

[0026] The reinforcing filler may be either carbon black as shown or silica or a blend of both. The total amount of filler present may be from 15 to 160 parts by weight per hundred parts by weight of styrene-butadiene rubber. Preferably the filler is present in an amount from 30 to 120 parts of the rubber. If silica is used, then this may be in conjunction with a coupling agent, preferably a silane coupling agent, e.g. bis 1,3-triethoxy-silyl propyl tetrasulphide. Silica may have a surface area 60-300 m²/g, preferably 80-250 m²/g and is preferably precipitated silica. The coupling agent may be used in the amount from 2-18.5% by weight of silica filler, preferably 5-12% by weight.

[0027] The carbon black may be a high surface area carbon black of SAF or ISAF type, having a surface area 80-300 m²/g and be present at 15 to 160 and preferably 30 to 120 parts by weight per 100 parts by weight of styrene-butadiene rubber.

[0028] The oil in the composition is preferably an aromatic oil and may be present from 10 to 180 parts by weight per hundred rubber, preferably 15 to 120. Suitable aromatic oils are well known to a man skilled in the art. Preferably they have a specific gravity from 0.95-1.0.

[0029] The rubber compounds may contain conventional additives, e.g. accelerators, activators, extenders and anti-degradants as desired. The vulcanisation agent is preferably sulphur or a sulphur-containing compound.

[0030] The resultant tires have been found to have good precision under the slower speed involved in cornering and due to the lower temperature generation in the second rubber compound the tire also has improved durability at high speed running.

[0031] Whilst the presence of the interfacial fabric is preferred most of the advantages of the present invention can be obtained if it is omitted. TABLE 1 A B Solution SBR 145 145 Carbon black 90 109 Aromatic oil 44 44 Zinc oxide 4.0 4.0 Stearic acid 1.3 1.3 Anti-degradants 4.0 4.0 Processing aid 3.0 3.0 Accelerator 2.6 3.3 Sulphur 1.5 1.9 

1. A motor-cycle tire (1) comprising a ground contacting tread (2) reinforced between its edges (3,4) by a breaker assembly and having in its normally inflated fitted condition a camber value C/L of between 0.5 and 0.7, a reinforcing carcass ply (14) extending radially inside the breaker assembly and between two bead regions (10,11) and wrapped in each bead region around an annular bead core (12,13) from the axial inside to the outside to form carcass ply turn-ups (15,16) and between the tread edges (3,4) and bead regions (10,11), tire sidewalls (8,9) wherein the breaker assembly comprises two breaker plies (5,7) comprising reinforcing cords oppositely inclined with respect to the circumferential direction of the tire characterised in that the tread comprises two different rubber compounds, a first rubber compound extending in a continuous first layer between the tread edges (3,4) and a second rubber compound disposed in a second layer radially outwardly of the layer of the first rubber compound to form the tread running surface in the central portion of the tread, centred on the tire axial centreline, having a width (S) of between 30% and 50% of the tread width (TW), the widths being measured along the arc of the tread surface, and on an interfacial fabric layer disposed at the boundary between the first and second rubber layers, the cords of the fabric layer are laid parallel to each other and disposed at an angle of 18°-30° to the tread circumferential direction and radial thickness of the second layer of rubber is between 50% and 80% of the total radial thickness of the tread.
 2. A tire according to claim 1, characterised in that the interfacial fabric layer comprises nylon cords.
 3. A tire according to any of claims 1 or 2, characterised in that the radial thickness (t) of the portion of first rubber layer radially inward of the second layer is not less than 2 mm.
 4. A tire according to any of claims 1 to 3, characterised in that the hardness of the rubber of the first rubber layer (RL1) at 100° C. lies in range 40-49 Shore ‘A’.
 5. A tire according to any of claims 1 to 4, characterised in that the first rubber layer (RL1) has a tensile modulus at 300% extension at 100° C. of less than 5.0 MPa.
 6. A tire according to any of claims 1 to 5, characterised in that the rubber of the second rubber layer (RL2) has a hardness at 100° C. of not less than 500 Shore ‘A’.
 7. A tire according to any of claims 1 to 16, characterised in that the rubber of the second rubber layer (RL2) has a tensile modulus at 300% extension at 100° C. of not less than 5.0 MPa. 